1. Field of the Invention
This invention relates to nuclear reactors and more particularly to a nuclear core for a water moderated nuclear reactor.
2. Description of the Prior Art
In the past, fuel elements and hence, nuclear cores have been advanced having differing amounts of fissile plutonium mixed with enriched uranium-235 for use in thermal or "slow" reactors. However, the actual usage of plutonium-239 in nuclear reactors of this type, for example, a water moderated nuclear reactor, has been particularly nil. The foremost reason being that fissile plutonium-239 simply does not exist in a natural state in those quantities which would be required for reactor usage. Another reason is that plutonium-239 which is produced as a fission by-product, that is, the plutonium generated during reactor operation, has not hithertofore been made available for commercial power generation. Still other reasons for the non-use of fissile plutonium involve the deleterious effects of the physical presence of plutonium within cores of thermal reactors. Some of the more important of these effects are described below:
The high neutronic capture cross-section of plutonium-239, as compared to uranium-235, has a negative influence on control rod worth. Since plutonium is more of a neutron poison that uranium, it suppresses the neutron flux thereby lowering the worth of the control rods.
The strong moderator temperature coefficient of plutonium results in a more negative temperature coefficient of reactivity which increases control requirements.
A sharp power peak occurs at interfaces of plutonium with uranium when the two nuclear fuels are combined in other than a homogeneous mixture.
In the near future, it is expected that significant quantities of fissile plutonium will become available due to the large number of water moderated commercial nuclear power plants now in operation, or near operation, as well as future fast breeder nuclear reactors. Since fissile plutonium is generated within these reactors as an irradiation by-product, it is less expensive than enriched uranium-235; accordingly, its use is highly desirable. However, before this economic advantage may be realized it is necessary to be able to negative the heretofore described adverse affects of the physical presence of plutonium within nuclear cores. No such solution exists in the prior art.